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I have 1981 luv diesel 4x4 does anybody know wy it could be almost sounds like backfireing.It does it when get it too about 55mph but wont go any faster.Thanks for any info

Sounds to me like it could be fuel starvation. Give us some background information, did this happen all at once, have you changed anything, bought fuel at a different place or loan it to your brother in law?
Oh, and do you have a service manual.


Fitzworld

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Fitzworld
A driveway full of Full size American 4x4 iron and one
LUV Diesel long bed

It has happened ever since i got the truck. I thought the normal things replaced fuel filter,replaced air cleaner,put fuel sysytem cleaner, and nothing helped. It used to do it at about 65 but here lately you cant even get it to 55 hardly without starting to backfire and smoking(black and blue).And i do have a service manual for it as well. Thought maybe fuel quality could be it but it doesn't matter where you go.Also thought could be water so i drained the tank and cleaned there was very little water in the tank also checked and drained water out of filter with no results. It honestly almost sounds like a popcorn popper when it does it.I honestly think it is fuel startvation as well but I have replaced lines to see if that would help it doesnt.Unless maybe there is a secondary pump before injection pump or something my manual that i ordered from chevy doesnt say anything about it. Let me know if you need anything esle Thanks.
I dont like my truck not running

Also just a note it does great in terms of oil usage doesnt use hardly any.But the fuel mileage has went down the drain.

Anything is possible but usually three things will cause backfire. Lean fuel mixture (poor delivery to one or more cyclinders), late timming or burnt valves. I would recommend looking at your injection pump to see if it has moved. Next run a compression test and if nothing bad shows up, then break out the service manual and perform a pressure test from the injector pump to the injectors and if that meets specs you will need to clean the injectors (or have them tested for flow).
Your problem should reveal itself in these tests, let us know of your progress. Good Luck

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Fitzworld
A driveway full of Full size American 4x4 iron and one
LUV Diesel long bed

Alright will try those things thanks

Just so you guys know finally found out what was wrong with my truck seals in injection pump are bad....Ordered seal kit for it should have it next week sometime so my truck will be back up and running the way its supposed to be again